The .25 - 35 Zischang does have a lateral motion safety built into it. As a hunting rifle which might be carried loaded, it makes sense. If this were a target rifle, it would be unneeded.
There is a pin to prevent barrel rotation or removal through the action. It passes through a "notch" in the bottom of the threaded shank of the barrel. I think that is the "screw" you refer to. These are quite common in Schoyen or Peterson built rifles. Having fairly easily removable barrels made a lot of sense when travel was by train or small coach and thus aided in making the rifle much more compact. A few of these rifles have multiple barrels, but the majority just seem to be to facilitate take down for transport.
